For the next 15 days, Posterous will announce a different service daily that will allow you to transfer your account, blog, videos, images and more over to the simple blogging site for free. First up: Ning. To switch, you simple give Posterous the URL of the Ning blog you want to move and your email address and Posterous will send you an email when they are done copying a site. You don’t have to have a pre-existing account with Posterous either to make the switch.

The switch over times will vary based upon size of site that is being converted, says co-founder Sachin Agarwal. Ten posts will only take a few minutes, but 1000 posts and large amounts of video and images (which Posterous will host) will take longer. The best part is that all of this work is done for you for free. And in case you don’t buy Posterous’ claims, here’s a testimonial page of beta users who have used the importing tools.

As for the platforms that Posterous will now offer switching from, Ning makes sense considering that many Ning owners who were using the network for free may be looking to switch after Ning shuttered its free service. Posterous is a nice alternative, with its enhanced customization features.

So which service is next in the switching plan? Agarwal and VP of Marketing Rich Pearson wouldn’t tell me but I think it’s safe to assume some of the platform that Posterous will target. While Posterous unveiled a “quick and dirty” converter for WordPress last year, Agarwal says it was fairly simple. It’s safe to assume that the startup will be launching a more comprehensive migration tool in the next two weeks. And it’s also safe to assume that TwitPic, Google’s Blogger, TypePad and Tumblr are also included in the lineup.

For Posterous, this is certainly an impressive campaign. Agarwal and Pearson say that this has been something the team has been working on for months. Clearly, Posterous has ambitions of becoming the defacto posting platform and is now making it significantly easier for users to switch. While the company doesn’t reveal number on how many blogs it hosts, Pearson saud that the site is growing in userbase by 20 to 25 percent each month. A little aggressive marketing could speed that growth up significantly.